Reciprocating engines have been in a constant state of evolution ever since their initial conception. The uses and applications of a host of different designs have appeared to be limited only by the ingenuity and needs of designers and engineers. A somewhat recent development included incorporating the reciprocating pistons in undersea torpedoes. Aluminum and alluminum alloy pistons work satisfactorily in some torpedo designs. They have, however, a relatively short operational life since the intense heating causes melting and erosion in the upper portion of the piston. This has been noted in many practice torpedoes since they routinely are overhauled and the pistons changed after only five torpedo runs. This is an unduly time consuming and costly procedure and should be avoided if possible.
Historically, heat resistant coatings have long been applied to pistons to extend their useful life. The U.S. Pat. No. 2,833,264 to J. J. Daly et al sprays a molten molybdenum coating onto an aluminum piston to enhance heat transfer. M. G. Whitfield et al in U.S. Pat. No. 3,152,523 cast a titanium cap onto an aluminum piston with the hopes of withstanding the high operating temperatures. A still later attempt to improve piston life for an internal combustion engine is shown in U.S. Pat. No. 3,596,571 to L. T. Hill et al. Aluminum-copper-nickel-manganese alloy caps, sintered aluminum caps, copper alloy caps, nickel alloy caps or steel caps are friction welded onto an aluminum piston. The caps provide for increased heat resistance and can be fashioned to help define an internal cooling cavity that helps dissipate heat. Robert D. Downell in U.S. Pat. No. 3,911,891 deposited a nickel alloy layer, a combination nickel aluminum alloy and refractory zirconium oxide layer, and a zirconium oxide layer onto the head of a piston using a plasma flame spray process. This laminate on the piston head is said to enable operation at greater temperatures than an untreated head. Another plasma sparying technique shown in U.S. Pat. No. 3,914,574 applies a wear resistant coating of cast iron, chromium, molybdenum or nickel or grooved surfaces of a two-part piston. Still another piston fabrication technique is shown in U.S. Pat. No. 4,334,507 and casts an aluminum or aluminum alloy body on a higher stress withstanding material such as a sintered chromium nickel steel, nickel chromium alloy, nickel and nickel iron alloy or copper or bronze. This combined material selection is said to increase mechanical bonding between them to produce a stronger piston. While this listing of fabrication techniques is not intended to be exhaustive, it is typical of the variety of approaches which have been relied upon in the past to improve the lifetime expectancy of aluminum and aluminum alloy pistons. All have contributed to advancing the state-of-the-art yet optimization and further improvements are needed to meet the requirements for higher operating temperatures for longer periods of times as demanded by an emerging generation of torpedoes.
Thus, a continuing need in the state-of-the-art exists for a method and apparatus for improving the life expectancy of aluminum pistons included in current high performance torpedoes.